Binary asteroid
A binary asteroid is a pair of asteroids orbiting each other, often a large body with a small moon.
A binary asteroid is two asteroids bound by gravity and orbiting a common centre. The first confirmed asteroid moon was Dactyl, found orbiting Ida by the Galileo spacecraft in 1993. Since then, hundreds of binaries have been identified, mainly from the way their combined brightness dips as one passes in front of the other.
About one in six near-Earth asteroids larger than a few hundred metres is thought to be binary. Many form when a fast-spinning rubble pile sheds material from its equator, which gathers into a small moon. Didymos and its moon Dimorphos, the target of the DART test, are an example.
Binaries are useful because the moon's orbit reveals the total mass of the system, and from that, the density. Low densities show that many asteroids are loosely packed rubble piles with lots of empty space inside. A binary asteroid striking Earth could make two craters side by side, and a few such crater pairs have been proposed.
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Level: Explorer Ages 10 and up